---
title: "Confidence Interval Calculator — Mean & Proportion (90%, 95%, 99%)"
description: "Calculate confidence intervals for a population mean or proportion at 90%, 95%, or 99% confidence. Automatically uses t-distribution for n < 30. Explains US vs UK vs IB curriculum differences."
url: "https://worldcalculators.org/calculators/confidence-interval/"
canonical: "https://worldcalculators.org/calculators/confidence-interval/"
---

# Confidence Interval Calculator

Calculate confidence intervals for a population mean or proportion at 90%, 95%, or 99% confidence. Automatically uses t-distribution for n < 30. Explains US vs UK vs IB curriculum differences.

## Quick answer

The 95% confidence level (α = 0.05) was established largely by Ronald Fisher in the 1920s as a pragmatic threshold. It became embedded in research practice globally through statistics textbooks and journal reporting standards. The American Psychological Association (APA), British Medical Journal (BMJ), and most international journals require reporting 95% CIs. Some fields (particle physics, drug safety) use 99% or even 99.9999% (5-sigma) due to the consequences of false positives.

## Frequently Asked Questions

The 95% confidence level (α = 0.05) was established largely by Ronald Fisher in the 1920s as a pragmatic threshold. It became embedded in research practice globally through statistics textbooks and journal reporting standards. The American Psychological Association (APA), British Medical Journal (BMJ), and most international journals require reporting 95% CIs. Some fields (particle physics, drug safety) use 99% or even 99.9999% (5-sigma) due to the consequences of false positives.
US FDA: requires 95% CIs for efficacy endpoints, typically two-sided. EU EMA (European Medicines Agency): also uses 95% CIs, but may require 99% for safety data. Both require pre-registration of confidence level before the trial begins. NICE (UK) for health technology appraisals uses 95% CIs and incremental cost-effectiveness ratios (ICERs). The underlying mathematics is identical — only regulatory thresholds and reporting requirements differ.

Source: https://worldcalculators.org/calculators/confidence-interval/
Markdown mirror of the HTML page. Prefer this URL for RAG; the interactive calculator still lives on the HTML page.